1. Field of the Invention
The present invention generally relates to the field of fabricating microstructures, such as integrated circuits, and, more particularly, to the deposition of silicon nitride layers by chemical vapor deposition (CVD).
2. Description of the Related Art
During the fabrication of microstructures, such as integrated circuits, one important process technology is the deposition of dielectric films for various purposes, such as electrically insulating certain device areas, forming a dielectric for capacitors, providing a barrier function against diffusion of undesired materials, creating a desired etch selectivity with respect to other materials, and the like. Especially, in the field of integrated circuit fabrication, a large amount of presently available products is based on silicon, and, hence, the dielectric materials silicon dioxide and silicon nitride are frequently used for the above-identified purposes due to their superior compatibility with silicon. While silicon dioxide is frequently used as an insulation layer separating, for instance, conductive silicon regions from each other due to the thermal stability of the dioxide/silicon interface, silicon nitride is often used as a barrier material, an etch stop layer for patterning a silicon dioxide layer or a stop layer during the chemical mechanical polishing of silicon dioxide. Silicon nitride (Si3N4) is a hard, dense, refractory material, the structure of which is quite different from that of silicon dioxide. Silicon nitride is harder, has higher stress levels and cracks more readily than silicon dioxide owing to the differences in structure between the silicon/nitrogen bonds and the silicon/oxygen bonds. Due to the dense structure of silicon nitride, other atoms and ions diffuse very slowly through silicon nitride. As a result, silicon nitride is frequently used as a barrier material to prevent the oxidation of underlying layers or the diffusion of metal ions, such as copper, through sensitive device areas. Moreover, silicon nitride layers are frequently used as etch stop layers in both wet etch and plasma etch processes. Even though silicon nitride usually contains a moderately high amount of hydrogen, which may reach up to approximately 20 atomic percent for silicon nitride deposited by plasma enhanced CVD and several atomic percent for silicon nitride deposited by thermal CVD, the diffusion of hydrogen is also effectively hindered by the dense structure of silicon nitride. The characteristics of silicon nitride, such as its hardness or the optical properties represented by its refractive index, significantly depend on the specifics of the deposition process, especially when a plasma enhanced CVD process is employed, so that desired layer characteristics may be obtained by correspondingly selecting the process parameters for a deposition recipe. For instance, a varying amount of oxygen may be provided during the deposition of silicon nitride so as to obtain a silicon oxynitride, the properties of which may be fairly continuously varied from those of the pure oxide to those of the pure (hydrogenated) nitride.
From the above, it is evident that silicon nitride layers may serve a plurality of purposes in fabricating microstructures and hence corresponding process tools and process recipes have been established to provide silicon nitride layers with a required layer thickness and with desired mechanical, electrical and optical characteristics. A typical deposition regime for producing a silicon nitride layer is chemical vapor deposition, wherein one or more substrates are introduced in a process chamber and exposed to a deposition atmosphere in which appropriate precursors react under specified thermal conditions so as to form a silicon nitride layer with specified characteristics on the substrate. Frequently used precursors are, for instance, dichlorosilane (SiCl2H2) or silane (SiH4), wherein ammonia (NH3) or pure nitrogen is used as an oxidant. Typically, a temperature is within approximately 700-800° C. at a pressure of approximately 1 Torr. It turns out, however, that silicon nitride thermal CVD processes with temperatures below 600° C. are unavailable and therefore plasma enhanced chemical vapor deposition (PECVD) has become a standard technique for forming silicon nitride layers from the above-specified precursors at temperatures in the range of approximately 200-400° C.
FIG. 1 represents a schematic sketch of a deposition tool that may be used for plasma enhanced CVD deposition for forming various layers, especially for forming a silicon nitride layer on a substrate. In FIG. 1, a deposition tool 100 comprises a process chamber 101, containing therein a shower head 102, connected to a precursor supply line 105 and to coolant supply and exhaust lines 106. The lines 106 may be connected to an appropriate coolant source (not shown) so as to allow temperature control of the shower head 102 during the operation of the deposition tool 100. The precursor supply line 105 is connected to a precursor source 107, including a plurality of precursor gases that are suitable for forming a silicon nitride layer. For instance, the precursor source 107 may comprise a source of dichlorosilane or silane for delivering silicon and an ammonia or nitrogen source as an oxidant or reactant. Furthermore, the precursor source 107 may comprise a precursor for reactive etch chemistry on the basis of hydrogenated fluorides (HF) or carbon fluorides (CxFy) so as to establish a cleaning atmosphere within the process chamber 101, as will be described in more detail later on. The shower head 102 may comprise an upper electrode 104 connected to a plasma excitation means 108, which is also connected to a second electrode provided in the form of a substrate holder 109. The shower head 102 further comprises a porous surface 103 having formed therein a plurality of passageways so as to substantially uniformly provide precursor ions and molecules to a substrate 110 disposed on the substrate holder 109. Typically, the substrate holder 109 further includes a heater 111 in thermal contact with the substrate 110, thereby allowing a certain degree of temperature control of the substrate 110 during operation. Finally, the process chamber 101 comprises outlets 112 that may be connected to appropriate vacuum sources, such as vacuum pumps, to discharge deposition byproducts and to establish a required pressure within the process chamber 101.
During operation of the deposition tool 100, a required amount of precursor gases is fed to the shower head 102 by controlling the respective flow rates of each of the precursor gases, wherein the plasma excitation means 108 is energized so as to provide a respective AC power to the electrode 104 and the substrate holder 109, thereby generating a certain amount of reactive ions and molecules that are distributed relatively uniformly across the substrate 110 by means of the porous surface 103. Without describing any details of the deposition kinetics, it should be noted, however, that the finally achieved characteristics of the silicon nitride film deposited on the substrate 110 strongly depend on the process parameters, such as the geometry of the shower head 102 and the process chamber 101, the temperature of the shower head 102, the temperature of the substrate 110 (which might not necessarily be identical to the temperature of the heater 111), the pressure within the process chamber 101, the flow rates of the individual precursor gases, the RF power supplied by the plasma excitation means 108, and the like. As previously explained, substantially uniform characteristics of the silicon nitride layer should be obtained across the substrate 110 and should also be obtained for a plurality of substrates that are processed subsequently in the deposition tool 100. That is, mechanical, electrical and optical characteristics, as well as the finally obtained layer thickness of the silicon nitride layer, have to be as uniform as possible for a plurality of substrates so as to meet the restrictive requirements of modern fabrication techniques of microstructures.
During operation of the deposition tool 100, it may be necessary to clean the process chamber 101 on a regular basis so as to provide substantially similar processing conditions for a plurality of substrates. During the deposition of the silicon nitride layer, non-negligible amounts of silicon nitride and possibly of other byproducts may also be deposited on the chamber walls of the process chamber 101. These byproducts may accumulate over time and significantly influence the processing conditions within the chamber 101 for subsequently processed substrates. For example, an increasing layer thickness deposited on the chamber walls may change the radiation behavior of the walls, thereby possibly sensitively influencing the amount of heat transferred to the substrate 110 by heat radiation. Since the actual surface temperature of the substrate 110 may significantly influence the layer characteristics and/or the deposition rate, substrate-to-substrate uniformity may deteriorate as the number of processed substrates increases. Moreover, since high energetic particles may also hit the chamber walls during the plasma enhanced deposition process, material accumulated on the chamber walls may be sputtered off and may reach the substrate 110. When the accumulated material reaches a certain thickness on the chamber walls, larger sized particles may delaminate and hit the substrate 110, thereby contaminating certain areas of the substrate 110.
It is, therefore, common practice to periodically clean the process chamber 101 by plasma etching with appropriate cleaning chemicals, thereby effectively removing deposition residue from the chamber walls and the chamber's components, especially from the porous surface 103, so as to reduce the risk of clogging the passageways. The plasma cleaning process may be performed after the processing of a plurality of substrates or may even be performed for each individual substrate. Depending on the selected cleaning regime, a more or less extended time interval may be required to remove the deposition residue. Since the cleaning process typically requires other parameter settings, for instance, with respect to flow rates, pressure chamber, and the like, compared to the actual deposition process, a variation of the layer characteristics may be observed for substrates processed immediately after the plasma cleaning and conditioning compared to substrates processed without a preceding cleaning process. The same holds true for any non-deposition periods of the tool 100, for instance, for any idle periods when the tool 100 is maintained in a standby mode or when a wafer transfer event occurs. Especially in deposition tools having a plurality of shower heads and substrate holders for simultaneously processing a plurality of substrates, frequent wafer transfer activities may contribute to a subtle destabilization of process conditions. Thus, typically, a plurality of test substrates must be processed prior to the processing of actual product substrates so as to maintain the thickness variation of the silicon nitride layers within specified process margins, thereby reducing productivity and tool utilization.
In view of the above-identified problems, there exists a need for an improved technique for operating a deposition tool for producing a silicon nitride film, in which, among other things, the thickness uniformity from substrate to substrate is improved.